The human eye notices contrast. In other words, the color of the button won't matter as much as the way the color differs from your overall design. If most of your site is green, using a green call to action button would be a mistake. Choose a color that creates contrast.
All colors send unconscious signals to the minds of people. Red stands out perfectly, provided that most of your website's design isn't red. However, it can make people unconsciously think that your website is not safe.
Yellow can be noticed very easily, however, it's an irritating color. Some fast food restaurants paint their chairs yellow to make people leave quickly and to free more room for new customers.
Orange is the perfect combination of Red and yellow. It can be noticed extremely fast and it doesn't send unconscious negative signals. This is why it's believed that orange could be the best color for a call to action button.
Many studies have shown that red is better at conversion than green. But the problem with these studies is that they didn't take the site's color design into consideration.
In order to know what color works best for your site, design two similar pages and do some A/B testing. Fix everything on the pages except the button colors, and find out which color converts the most.
Some people claim that unconventional looking buttons catch attention faster. This makes some sense because the brain usually ignores repetitive data and focuses only on what might be interesting.
Amazon.com, the large online retail giant, uses an orange button and they do it for a reason. They believe the orange color provides best conversions. However, had they removed some of the orange in their site's design their conversions might increase (See how Amazon started).
One of the things you can do to increase conversions without worrying about colors is increasing the size of the button you are currently using. However, make sure you don't increase the size of the button to a point where customers feel that you are pressuring them to buy.